Electrical Engineering - Chapter 1

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EE 101 Electrical Engineering 2001/02 Overview

University of Moratuwa - JRL/Oct2001 1

EE 101 Electrical Engineering (2 Credits)

taught by Rohan Lucas, Jahan Peiris, Anula Abeygunawardena

Syllabus

Overview (4 hrs)

Electrical Power and National Development, Role of Electrical Engineer, Introduction to PowerGeneration, Transmission, Distribution and Utilisation including Modern Drives, SI Units, Basic

concepts.

Network Theorems (4 hrs)

Ohms Law, Kirchoffs Law, Superposition Theorem, Thevenins theorem, Nortons theorem,

maximum power transfer theorem, Millmanns theorem; star-delta transformations. Introduction to

nodal and mesh analysis..

Alternating Current theory (6 hrs)

Sinusoidal waveform, phasor and complex representation, Impedance, Power and Power factor.

Analysis of simple R, L, and C circuits using alternating current Solution of simple network

problems by phasor and complex number representation.

Three Phase - Advantage of three phase, Star and Delta configurations. Phase sequence. Balanced

and unbalanced systems. Power factor correction.

Electromagnetic and Electrostatic theory (2 hrs)

Basic electrostatic and electromagnetic theory; force and torque development in magnetic circuits.

Electrical Measurements (3 hrs)

Direct deflection and null deflection methods. Ammeters, voltmeters, wattmeters, energy meters.

Extension of ranges.

Electrical Installations (3 hrs)

Fuses, miniature circuit breakers, earth leakage circuit breakers, residual current circuit breakers,

earthing, electric shock. IEE wiring regulations, basic domestic installations.

1.0 Overview

You may find it amusing that this lecture series starts with a cartoon

which appeared in the magazine Punch in London on 25th

June

1881(Figure 1.1). It shows King Steam and King Coal discussingBaby

Electricitys chances of success in life. Baby electricity is obtaining

nourishment from the feeding bottle marked storage of force. Prior to

1881, Coal gas was widely used to obtain light and steam was used to

drive machinery and as such the advent of baby electricity into this

market made King Coal and King Steam become anxious about their

future.

It was around this time that electricity started gaining ground for lighting

and for power applications. The success was based primarily on the

discovery of electromagnetic induction and the construction of the firstdynamo by Michael Faraday in 1831.

From ancient times, the use of non human forms of energy transfer has

been associated with the development of man. For example, stone age

man used spears to hunt animals for food. However he had to be close

enough to be able to succeed. But this had a lot of dangers. So he

invented the bow and arrow.

The bow is slowly bent by the hunter in more than a few seconds and energy is stored in the springy wood.

However it is released in a fraction of a second on to the arrow so that it moves at many times the velocity that the

man could have thrown at, and so can operate it in from a much further distance. [Note that the original muscle

energy in man was converted to potential energy in the bow and kinetic energy in the arrow. The man in turn gets

his energy from the food - principle of conservation of energy]. Animal power was also used by man to materially

improve himself.

Figure 1.1 What will he Grow to ?


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Water power in water wheels and wind power through wind mills were the first forms of non-muscle power to be

used. The invention of the steam engine in the 1780s was a real break through in that it lead to an ever increasing

material progress.

With the advent of large scale electrical generation and transmission about 100 years later, the rate of increase of

power has been rocketing. Although this is wonderful for our material prosperity, the machines too have to be fed

and our limited resources of fossil fuel, as is used today, are fast dwindling.

The standard of living of a country can be measured by the amount of energy consumed per person ( per capita

consumption). While many developed countries need energy to heat the environment during the harsh winter weare fortunate not to have that requirement. Even so our per capita consumption of energy is still too low for us to

become an industrialised country.

Engineering economics play a vital role in the development of a country. The role of the electrical engineer is to

facilitate the means by which this can be achieved through the proper management of resources. His role is to

economically design, operate, maintain and manage the equipment associated with the generation, transmission,

distribution and utilisation of electrical energy and at the same time maintaining safety.He should also be able toadequately communicate with the lower and higher rungs of the organisation, and be able to relate to the

environment. He must realise that electrical engineering is an ever expanding field and that he needs to continually

develop himself to meet the needs of society and that industrial problems rarely have a well defined or unique

solution.

Generation, transmission and distribution are directly concerned with electrical power and are handled by supply

authorities. Utilisation is by all walks of people for all types of purposes. The electrical equipment would consist

not only of purely electrical equipment such as for lighting, heating, machinery, and transport but also for other

applications such as computers and communication equipment.

In the past over 90% of motors used in industry have been traditional induction motors. In the present day, more

and more power electronic devices are used to control motors.

1.1 Generation, Transmission and Distribution of Electricity

The consumer needs to use electricity for his day to day activities in his home, office or other workplace. It is

neither economical nor feasible to produce electricity (i.e. convert from non electrical form of energy to electrical

energy) where it is required. Thus electricity is produced centrally in generating stations. In the home or office,

we would normally use electricity at 230 V (110 V is used in the US) between live and neutral wires phase

voltage. Voltages higher than that would not be at all safe and voltages lower than that would not be economical

as they require larger currents and hence larger size conductors [Note: for the same power output, a lower voltagewould require a higher current]. We would also have a protective earth wire to give us additional safety.

The supply is an alternating current at a frequency of 50 Hz. A typical house in Sri Lanka would normally not

consume more than about 2 to 3 kW of power even during the peak giving a peak current of about 10 A. Since

there are a number of houses down your lane and all of them require electricity a common distribution line is run

along the side of the road and power is tapped for each house from it using 2 wires (live and neutral) and the earth

wire is provided from an electrode at your premises. When the current flows, there will be a considerable power

loss in both the live and neutral wires and a considerable voltage drop [Note: voltage drop is different from the

voltage of the line]

You are familiar with the fact that forces along three sides of an equilateral triangle add up to zero. Likewise,

three currents, equal in magnitude but with phase angles of the sinusoidal waveform differing by 1200

(or 2/3rad), would add up to zero.

Figure1.2 - Three phase waveforms

-100

-50

0

50

100

0.000 1.571 3.142 4.712 6.283

angle (rad)current

R phase Y phase B phase

Making use of this property, if we could balance the electrical loads taken by the various households equally into

three phases and supply them at different phase angles (as shown in figure 1.2), we would have almost no neutral

current. Thus the total power loss would be effectively halved. Therefore power distribution is normally carried

out using three phase.


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Using trigonometry, it can be easily shown (figure 1.3) that the voltage

between two live wires would now not be 230 V but 2 230 3/2 or 400V. [The voltage between live and neutral is still 230 V]. Thus the low voltage

distribution from which supply is obtained to your house is called 400 V, 3

phase, 50 Hz supply. The house in turn has a 230 V, 1 phase, 50 Hz supply.

When we consider all the houses down your lane, and perhaps the adjacent

lane, the total current may approach around 500 A per phase and would

require a power supply of the order of 350 kVA.

Since the power loss in a line is proportional to the square of current (I2R), the loss rapidly increases with

increase in current or power distributed (VI). This power loss may be reduced by sending the power at a higher

voltage as power loss is proportional to the square of current, while power distributed is proportional to the

current.

Thus power is distributed at a higher voltage such as 3 phase, 11 kV (Lanka Electricity Company) or 3 phase, 33

kV (Ceylon Electricity Board). Big industries (such as Steel Corporation) and large workplaces (such as

University of Moratuwa) are supplied at these voltages. The voltage is stepped down from these voltages to 3

phase, 400 V using transformers (usually rated at 100 kVA, 250 kVA, 400 kVA etc) for low voltage distribution.

Thus there are two levels of distribution - low voltage and medium voltage (figure 1.4).

Generating stations (ex. Kotmale, Victoria, Laxapana) are commonly located hundreds of kilometers away from the

Load Centre (ex: Colombo). Also there would be thousands of transformers supplying the low voltage distribution

systems. Thus it is not economical to transmit over long distance at 11 kV or even 33 kV. Also, in such

transmission, it is point to point (such as from Kotmale to Kolonnawa) and in bulk. Thus high voltage transmission

is carried out in Sri Lanka at 132 kV and more recently at 220 kV to keep the power losses low. The powertransfer would be of the order of 100 MW. [For a given conductor size and power transfer, an increase of voltage

by a factor of 20 times would reduce the current by a factor of 20 times and the power loss by a factor of 400

times].

In some countries, where the power transfer requirements are much higher and the distances much longer, extra

high voltage transmission at voltages higher than about 345 kV (ex: 400 kV, 760 kV etc) are used.

Unfortunately, it is not feasible at present to generate electricity at more than about 25 kV (A Powerformer is being

developed in Sweden capable of generating at high voltages required for transmission) so that the normal

generation is done using Synchronous Generators at about 11 to 15 kV (typically 13.8 kV). This is stepped up to

the transmission voltage using transformers. The transmission voltage in Sri Lanka is 132 kV and 220 kV.

Figure 1.3

Generating Station13.8 kV

Transmission Line220 kV

Transmission Line132 kV

Distribution Lines33 kV

Load

33 kV, 3

Loads

230V, 1

Loads230 V,1

Distribution Line, 400 V

Distribution Line, 400 V

Distribution Lines

11 kV

Loads

400 V,3

Loads400 V, 3

Figure 1.4 - Typical arrangement of part of a power system

230 V

400 V

300120

0

Red

YellowBlue


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1.2 Bulk Energy Sources

Energy sources for bulk power generation can be categorised into renewable energy sources (ex: hydro-electric,

solar, wind etc) and non-renewable energy sources (coal, oil etc). Some of the more common ones are outlined in

the following section.

(i) Hydro-electric power

Perhaps the oldest form of large scale energy conversion has been by the use of water power which were used in

ancient water wheels. In the modern hydro-electric power stations, the energy of water (either in potential formor kinetic form) is first converted to rotational (mechanical) energy in the turbine and then to electric energy in the

generator. Although the water which gives the energy is free, a high capital civil engineering cost is associated

with the construction and the overall cost per kWh may approach that of conventional thermal plants.

Micro-hydro and mini-hydro plants are also used and connect to the distribution network directly rather than to the

transmission network.

(ii) Conventional thermal power (Coal & Oil)

The burning of coal or oil in boilers produces steam at high temperature and pressure (conversion of chemical

energy to thermal energy and then to kinetic energy). This steam is used to rotate a steam turbine (thermal energy to

mechanical energy conversion) which is coupled to an electric generator (mechanical energy to electrical energy).

Even though continual advances have been made the efficiency of thermal stations is quite low (around 40%)

because of the vast quantities of heat lost to the surroundings.

(iii) Gas turbinesGas turbine plants also operate by burning oil. However the energy transfer is not through steam but through

compressed gasses produced in the combustion chamber under high pressure which drives the gas turbine. The gas

turbine is mechanically coupled to the electrical generator to produce electricity.

(iv) Nuclear Power Plants

The energy source in a nuclear power plant is the energy stored in the nuclei of the atoms of the fuel. Energy is

released by the splitting of a nucleus in a process called fission. Energy released by the combination of two light

nuclei to form a single heavier nucleus is calledfusion. So far power has only been successfully obtained from the

fission reaction. Uranium is the commonly used fuel and U235

is fissile. When struck by slow moving neutrons its

nucleus splits into two substantial fragments with high kinetic energy (about a million times larger than in a

chemical reaction). The fast moving fragments hit surrounding atoms producing heat. The neutrons released in the

fission reaction cause further fissions and under correct conditions lead to a chain reaction. The chain reaction is

controlled by the use of reaction absorbers placed in control rods. The heat generated is used to produce steamwhich in turn drives a steam turbine coupled to an electrical generator as in a coal or oil power plant.

(v) Wind Power Plants

Wind is continuously regenerated in the atmosphere under the influence of the energy radiated from the sun. (about

20% of the solar energy of the atmosphere is converted to wind energy). The kinetic energy in the wind is used to

turn a wind turbine. The energy produced in a wind turbine is proportional to the cube of the wind velocity. Wind

turbines (usually 2 or 3 bladed) must be mounted on tall towers for greater efficiency as wind velocity is higher at

higher altitudes. To obtain large scale electric power, a large number of wind turbines are arranged in a wind

farm. One of the disadvantages of wind power is that it is not continuously available.

We have about 150-200 MW of wind potential in the coastal belt in Sri Lanka.

(vi) Solar Energy

Energy generated by the sun is due to a nuclear reaction called fusion. The energy that is received on the earthfrom the sun is over 10,000 times the present energy requirements of the earth. The average solar radiation

received by the earth is about 0.2 kW/m2.

In large scale installations, the suns rays may be concentrated by lenses or mirrors. These lenses or mirrors need

to have steering mechanisms to follow the motion of the sun and to focus the heat on solar towers. The heat can be

used to produce steam to drive steam turbines.

Photo-voltaic power using solar cells is also used to produce electricity. In these the solar radiation is directly

converted into electricity. At present solar cells are expensive and are not used for large scale generation of

power but may be used in some rural electrification schemes. However these require power storage as electricity

is not produced at night when it is most needed for lighting.

A very important use of Solar energy is in powering remote power applications such as repeater stations,

transmitters at hill tops, and in extra terrestrial applications.


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(vii) Geothermal Energy

Geothermal energy is the heat from the inside of the earth. The temperature of the earth crust increases from about

200C at the earths surface to about 1000

0C at a depth of 40 km. Energy can be obtained by drilling at suitable

locations to release high pressure steam or hot water which is produced by ground water coming into contact with

molten rock. These can be circulated through a heat exchanger to produce steam to run a steam turbine. In areas

where the ground water supply is limited, water could be pumped into a cavity created in the hot rock to obtain hot

water and steam.

(viii) Ocean Energy

Energy from the ocean can be developed in three different ways. These are (a) tidal power, (b) ocean waves and

(c) ocean thermal gradient.

In tidal power, we make use of the tide which occurs twice a day and rises and falls by as much as 10m. When the

tide rises, water flows in to the land and when the tide falls, water flows out. This vast amount of energy transfer

can be used to generate electrical energy using hydro turbines.

Ocean waves are created by the wind and a vast amount of energy is available in the waves. A kilometer long, one

meter wave would contain about 10 MW of power. Reciprocating generators are commonly used. The mechanism

to trap the wave energy would be moored in the middle of the ocean (say 80 km from shore).

Ocean thermal energy conversion (OTEC) uses the natural temperature difference between the warm surface water

(250C) and the cooler ocean bed at 5

0C. Due to the low temperature difference, the turbines use fluids such as

ammonia as the working fluid.

About 50 km away from the coast line in Sri Lanka, in Mannar, Trincomalee and Unawatuna, are some of the best

sites for OTEC in the world. A 1 MW OTEC plant is under construction near Mannar.

(ix) Magneto hydro-dynamic (MHD) Generation

Magneto hydro dynamic generation is also based on Faradays laws of induction. The main difference from a

conventional generator is that the conductor is a gas at a very high temperature and pressure where it is conductive.

The gas may also be seeded with a small amount of vaporised metal such as potassium.

In MHD generation, the gas passes through a pipe (whose cross-section gradually increases from beginning to end)

across which a magnetic field is applied (in a perpendicular direction to the flow of gas). An emf is thus induced

in a circuit in a third mutually perpendicular direction.

(x) Fuel Cells

Fuel cells use hydrogen-oxygen interaction through a catalyser to yield a flow of electrons in a load connected

externally. Each cell develops about 0.7V and sufficient modules in series yield an output voltage of around 11-15

kV and can reach power outputs of around 1 MW.

(xi) Biomass energy

Biomass is the abbreviation for biological mass, the amount of living material provided by a given area of the

earth's surface.Biomass energy may be obtained from forests, vegetation and animal refuse. In various forms, it isprobably the major supplier of energy in Sri Lanka.

1.3 Utilisation

(i) Electric Drives

Due to the various advantages of electric drives, they are universally used in industry. They may operate on either

alternating current (a.c.) or direct current (d.c.). Most applications use a.c. induction motors. However they

operate at almost constant speed and low starting torque. For variable speed drives and high starting torque

applications d.c. has been preferred. However, with modern power electronic controlled drives a.c. motors may

also be controlled relatively easily.

(ii) Electrical Heating

The heating characteristic of an electric current is used extensively in industrial and domestic heating applications.

They have relatively high efficiency (>75%), clean, easy to control, low maintenance and can be protected against

overheating easily.

Electric heating can be obtained from (a) resistance heating, (b) induction heating, (c) eddy current heating (d)

dielectric heating, and (e) electric arc heating.

(iii) Electric Welding

The word welding means the joining of two metals together by heating them to melting point. In electrical

welding, a very high electric current produces the heat needed to melt the material. Due to the reliability of the

welded joints in comparison to riveted or bolted joints, electrical welding has been adopted in many fields of

engineering. The types of electric welding are resistance welding and arc welding.


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(iv) Air Conditioning

By air conditioning, the temperature, humidity and the purity of the air are controlled. In tropical countries like Sri

Lanka, air conditioning only cools the air and not heats it, but in cold countries both modes are available. The

working substance in the cooling type of air conditioner is the refrigerant vapour which readily evaporates and

condenses.

(v) Electric Lighting

Light is electromagnetic radiation of a wavelength to which the eyes are sensitive (380 nm to 780 nm), bounded atone end by infra-red radiation of 780 to 10,000 nm and ultra-violet radiation of 10 nm to 380 nm wave length at

the other end. The colour associated with the various wavelengths are given in table 1.

Wave length

(nm)380-420 420-495 495-566 566-589 589-627 627-780

colour violet blue green yellow orange red

table 1

The eye is most sensitive to the colour yellow-green (wavelength = 555 nm).

Light sources are based on one of the following principles.

(a) Incandescent type - Incandescent filament type lamps emit light when heated to high temperature. Typical

incandescent lamps emit about 10 to 15 lm/W.

(b) Fluorescent type - Fluorescent lamps operate by transforming ultra violet rays into radiation of longerwavelength lying in the visible spectrum. Typical fluorescent lamps emit about 50-70 lm/W.

(c) Gas discharge type - Operate on the principle of visible radiation when an electric discharge occurs in a gas

or metal vapour. Sodium vapour lamp is a typical example of a gas discharge lamp and have efficacies of

about 100 lm/W.

(vi) Electric Traction

Two major application of electric traction are electric trains and electric hoists. The good controllability of speed

and torque of electrical drives without resorting to lossy gear systems is used in these applications. In addition to

this characteristic, the very high initial torque and almost constant power requirement in a vast range of speed

torque combinations of a DC series motor is frequently utilised. This allows the Internal Combustion (IC) engine in

a train, supplying power to the generator, to run at relatively low range of speeds, even at high running speeds of

the train. This enhances efficiency and cuts down wear and tear of the engine significantly.

(vii) Electrochemistry and Electrometallurgy

Electrical energy is extensively used in metallurgical and chemical industries for the extraction, refining and

deposition of metals and manufacture of chemicals. Though the various processes are different in apparent detail,

they are fundamentally alike, being based on the principle of electrolysis.

Extraction and refining of metals are similar electrochemical processes except that extraction is the term used for

the production of metals with commercially acceptable purity, while refining is the process by which a highly

concentrated of metals is subjected to electro-chemical treatment for recovering not only the principal metal in

pure form, but also the precious metals like silver gold etc which may be present in the form of minute traces.

(viii) Electronics

Electronics is the branch of science that deals with flow of electrons and other charged particles through gases,

vacuum and semiconductor materials. The principle electronic devices are the vacuum tubes, gas-filled tubes, and

semiconductor devices like selenium rectifiers, silicon diodes, transistors and silicon controlled rectifiers.

The applications of electronic devices and their circuit arrangements are numerous and varied

(ix) Information storage and transmission

The computer is the most powerful tool designed by man. Todays computer is a complex electronic machine

capable of storing, processing, manipulating and retrieving large volumes of data/information at incredibly high

speeds. Consequently they are finding ever increasing applications in every field of industrial, commercial and

business activity, science and space research, medical diagnostics etc resulting in greater production, higher

productivity, reduced costs and more reliable and accurate results.

(x)Biomedical Systems

Bioelectrical Engineering is the area of biomedical engineering concerning bioelectric activity, which

encompasses the nervous system and regulates most life processes. The bioelectrical engineer assists this

regulation and uses bioelectric signals for diagnostic purposes. Developments have led to the invention of the

pacemaker, the defibrillator, and the electrocardiograph. The monitoring of many other bioelectric functions bymeans of electrodes plays an important part in surgical recovery rooms and intensive-care units.


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EE 101 Electrical Engineering 2001/02 Electric Motors


1.4 Electric Drives

1.4.1 Electromechanical energy conversion:

Electrical Energy is obtained by the conversion of other forms of energy, based on the principle of conservation of

energy. The main advantage of this conversion is that energy in electrical form can be transmitted, utilised and

controlled more easily, reliably and efficiently than in any other form.

An electro-mechanical energy conversion device is one which converts electrical energy into mechanical energy

(electric motors) or mechanical energy into electrical energy (electric generators). The electro-mechanical energyconversion process is a reversible process. However, devices are designed and constructed to suit one particular

process rather than the other. In an energy conversion device, out of the total input energy, some energy is

converted into the required form, some energy is stored and rest is dissipated.

For practical application we need to convert electrical energy into the required useful form, such as mechanical

energy, heat, light, sound and electromagnetic waves. One of the major applications is electric drives which uses

motors.

Figure 1.5 - Power flow in an electric motor

The energy balance for a motor (shown in block diagrammatic form in figure 1.5) can be written as

Total electrical energy input = mechanical energy output + total energy stored + total energy dissipated

In a motor, energy is stored in the magnetic field and mechanical system. The energy dissipated is in the electric

circuit as ohmic losses, in the magnetic circuit as core losses (hysteresis and eddy-current losses), and in the

mechanical system as friction and windage losses. [ is the angular velocity of rotation].

An electric motor provides the driving torque necessary to keep machinery running at the required speed, with

provision being made for speed control and reversal of rotation. The electrical motor is required to start,

accelerate, drive and decelerate its load, within certain limitations.

There are three main types of motors - Direct Current Motor, Induction Motor and Synchronous Motor

Direct Current (dc) Motors

Direct current (dc) motors operate on a magnetic field produced by the field winding in the stator (stationary part

of the motor) interacting with the field produced by the armature winding in the rotor (rotating part). The basic

constructional features of a typical two-pole dc motor are shown in figure 1.6 and the circuit model in figure 1.7.

Figure 1.6 - two-pole dc motor Figure 1.7 - Circuit model of a dc motor

The dc motor needs slip rings or split rings (commutator) on the rotor shaft and a set of brushes positioned over

them to supply the armature winding. [symbol for the armature basically shows the rotor and a pair of brushes]

(a) separately excited (b) shunt excited (c) series excited (d) compound excited

Figure 1.8 - categorisation of dc motors

shaft

Electric MotorPstored, elect Pstored, mech

MechanicalLoad

Ploss,elect

Pinput

Ploss,mech

Poutput

dc supplydc supply dc supply dc supply dc supply

Vffield supplyarmature

supply

+

V

+Rf

raEaIf

+

Pmech V = Ea + ra Ia

Ia


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The dc motors can be categorised into four basic types dependent on the method of connection of the field winding

(figure 1.8). These are the (a) separately excited field, (b) shunt connected field, (c) series connected field, and

(d) compound connected field.

In the separately excited type, the field winding is connected to a separate or external dc source. In the shunt

excited type, the field winding is connected in parallel with the armature winding so that the same dc voltage

source is used. In the series excited type, the field winding is connected in series with the armature winding, again

making use of the same dc voltage source. Compound excitation involves both the series and shunt excited

windings.

In the case of very small motors, the field may be created by a permanent magnet rather than having a field

winding. These are known as permanent magnet dc motors.

Speed-torque characteristics of dc motors

The shunt, series and compound motors exhibit distinctive speed-torque characteristics, which are best suited for

specific tasks. Thus a study of motor characteristics is essential for one to decide on a specific application using

these motors. Figure 1.9 shows the speed-torque characteristics of dc motors. In addition, other characteristics

such as torque-current should be considered when selecting motors.

(a) shunt motor (b) series motor (c) compound motor

Figure 1.9 - Speed-torque characteristics of dc motors

DC motor applications

Although ac supplies are now universal, there are many applications, where the dc motor finds its place.

The dc shunt motor has a fairly constant speed against a varying load or torque. Thus, applications include

situations where a constant speed is required such as in lathes, conveyors, fans and machine-tool drives. Since the

dc series motor is able to create large torques at low speeds (high starting torque), it can be used to acceleratevery heavy loads from standstill. Thus, dc series motors are used for driving cranes, electric locomotives, group

drive shafts (where the motor is used as a drive for a whole assembly line), steel-rolling mills and so on.

Compound motors combine the characteristics of both shunt and series wound motors. The series winding gives

good starting torque and shunt winding ensures a comparatively constant speed. The actual characteristics of the

compound motor can be varied by varying the ratio of shunt to series field turns. They are used in applications such

as planers, shears, guillotines, printing machines and power presses that need peak loads at certain instances

(normally used with fly wheels to even out the load). Separately excited motors are used in applications where an

independent armature control and field control is required. Examples of their use are in steel and aluminium

rolling mills (high power) and control motors (low power).

In addition, permanent magnet motors are used for low power applications. These are specially used in

automobiles as starter motors, wiper motors, blowers in heaters and air-conditioners, in raising and lowering

windows. They are also used in such other applications as toys, electric tooth-brushes.

Stepper Motors

In certain applications, it is necessary to spin quickly and move to a precise point, such as in computer disk drives.

This can be accomplished by using a stepper motor. Stepper motors can be viewed as electric motors without

commutators. Typically, all windings in the motor are part of the stator, and the rotor is either a permanent magnet

or, in the case of variable reluctance motors, a toothed block of some magnetically soft material. The motors and

controllers are designed so that the motor may be held in any fixed position as well as being rotated one way or the

other. Most stepper motors can be stepped at audio frequencies, allowing them to spin quite quickly, and with an

appropriate controller, they may be started and stopped precisely.

AC Motors

With almost the universal adoption of the ac system of distribution of electric energy for light and power, the fieldof application of ac motors has widened considerably. There are a number of different types of ac motors, each of

which offers certain specific advantages.

differential

cumulative


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The supply for these motors is either three-phase or single-phase. Three phase motors are found in larger sizes

and have mainly industrial applications. Single-phase motors are used mainly for domestic and agricultural

applications. In the fractional kilowatt sizes, they are used in large numbers for washing machines, refrigerators

and so on.

AC motors are classified to various groups based on their principle of operation; most common are the induction

motor and synchronous motor.

Induction MotorsAs the name implies, the induction motor is based on the induced voltage in a winding in the rotor. The rotor does

not receive electric power by conduction but by induction in exactly the same way as the secondary of a

transformer receives its power from the primary. Of all the ac motors, the three-phase induction motor is the one

which is extensively used for various kinds of industrial drives.

An induction motor consists essentially of two main parts, namely (a) a stator and (b) a rotor (figure 1.10).

(a) stator (b) squirrel cage rotor (c) wound rotor

Figure 1.10 - rotor construction of induction motors

In the three phase induction motor the stator carries a 3-phase winding fed from a 3-phase supply. The stator

windings, when supplied with 3-phase currents, produce a magnetic flux which is of constant magnitude but which

revolves (or rotates) at a constant speed (at synchronous speed). On a 50 Hz supply, the synchronous speed for a

2-pole machine is 3000 rpm and for a 4-pole machine is 1500 rpm.

This revolving magnetic flux induces an emf in the rotor by mutual induction. This causes a current to flow through

the rotor conductors and produce a torque with the interaction of stator magnetic field. This torque causes the rotor

to start rotating to drive a load. The rotor-load combination settles at a lower speed than the synchronous speed

(typically around 2850 2975 rpm for a 3000 rpm synchronous speed) as power can be transferred only when

there is a relative speed. The difference between synchronous speed and rotor speed is termed as the slip. Slip

is usually defined as a ratio or percentage of the speed to the synchronous speed. Typically, slip is about 1-5 %.

s

s

N

NNs

speedssynchronou

speedrotorspeedssynchronouslip

=

= , .

The rotor of an induction motor can be of two types, namely (a) squirrel cage and (b) wound rotor as seen in

figure 6(b) and 6(c).

(a) Squirrel cage rotor

About 90% of induction motors are squirrel cage type, because this type of rotor has the simplest and most rugged

construction and is almost indestructible. The rotor consists of a cylindrical core with parallel slots for carrying

the rotor conductors which are not wires but heavy bars of copper, aluminium or alloys. The rotor bars are

permanently short-circuited at the ends to form the winding.

Because of the absence of moving parts in the circuitry, the motor is useful for duties in hazardous areas. It finds

applications for most industrial drives, where speed control is not required. These are specially used with loads

requiring low starting torque and substantially constant speeds. It can be shown that by increasing the effective

rotor resistance, the torque-speed characteristic can be modified such that the starting torque is increased.

However the operating slip also increases. With low rotor resistance, these are used in fans, centrifugal pumps,

most machine tools and wood working tools. With high rotor resistance, they are used in compressors, crushes,

reciprocating pumps etc. With very high rotor resistance, are used in punching presses, shears, hoists and

elevators.

(b) Wound rotor

Unlike the cage rotor type, the wound rotor type is provided with a three-phase winding in the rotor. Usually, the

three phases are connected internally as a star. The other three winding terminals are brought out and connected tothree insulated slip-rings mounted on the shaft with brushes resting on them. This makes possible the introduction

of additional resistance in the rotor circuit during the starting period for increasing the starting torque of the motor

and for changing its speed-torque/current characteristics.


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When running under normal conditions, the slip-rings are automatically short-circuited and the brushes lifted from

the slip-rings. Hence, it is seen that under normal running conditions, the wound rotor is short-circuited on itself

just like the squirrel cage rotor. Applications of the wound rotor type include high-inertia drives requiring

variable speed, fly wheel machine drives, air-compressors, ram pumps, crushing mills, cranes, hoists, winches and

lifts.

Typical Torque-speed characteristics of an induction motor are shown in figure 1.11. The effect of increasing the

rotor resistance on the characteristic is also shown.

Figure 1.11 - Torque-speed characteristics of an induction motor

Synchronous Motors

Synchronous motors operate on the same fundamental principles of electromagnetic induction as dc motors. They

usually consists of a 3-phase stator winding and a rotor winding which carries a dc current (figure 1.12). When the

3-phase stator winding is fed by a 3-phase supply, a magnetic flux of constant magnitude but rotating at

synchronous speed, is produced (as in 3-phase induction motors). This field interacts with the field produced by

the dc field winding on the rotor and produces a torque which can be used to rotate a load. It runs at a constant

speed (synchronous speed). However, the synchronous motor is not self-starting and hence needs additional means

for starting.

polesofpairspwherep

Ns == ;3000

Although the induction motor is cheaper for small power applications, the synchronous motor is preferred forapplications above 50 kW. Typical applications include Banbury mixers (used to mix raw ingredients for rubber

production), cement grinding mills, centrifugal compressors, mine ventilating fans, pumps, reciprocating

compressor drives, electric ship propulsion drives, large low head pumps, rolling mills, ball mills, pulp grinders,

etc. They are also used for power factor correction and voltage regulation.

Single-Phase Motors

Single phase motors are designed to operate from a single-phase supply and are manufactured in a large number of

types to perform a wide variety of useful services in homes, offices, factories, workshops, vehicles, air crafts,

power tools, etc.

Single-phase motors are usually classified based on their operating principle and method of starting, such as

(1) Induction motors (split-phase, capacitor, shaded-pole etc.)

(2) Repulsion motors (some times called inductive series motors)

(3) AC series motors

(4) Synchronous motors.

The single-phase induction motor is similar to a three-phase induction motor except that its stator is provided with

a single phase winding and a special mechanism employed for starting purposes (It does not develop a rotating

field but a pulsating field). It has a distributed stator winding and a squirrel cage rotor. Special mechanisms are

employed for starting and there are different motor types based on starting method such as split-phase (fans,

blowers, centrifugal pumps, washing machines, small machine tools, duplicating machines, domestic

refrigerators), capacitor start (small power drives) and shaded-pole (small fans, toys, instruments, hair dryers,

ventilators, circulators, electric clocks). Repulsion type motor applications include machine tools, commercial

refrigerators, compressors, pumps, hoists, floor-polishing and grinding devices, garage air-pumps, petrol pumps,

mixing machines, lifts and hoists. The ac series motor is a modified ordinary dc series motor that can be connected

to an ac supply. A similar one is the universal motor, which is a small version of ac series motor. This can workwith both ac and dc and used in applications such as vacuum cleaners, food mixers, portable drills and domestic

sewing machines. Single-phase synchronous motors are typically used in signaling devices, recording instruments

and in many kinds of timers and house hold electric clocks.

Figure1.12 - Synchronous Motor

Electrical Engineering - Chapter 1

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